1. Kilovoltage X-ray Dosimetry

2. Kilovoltage X-rays
Nomenclature and tube potentials of kilovoltage machines
Contact beams - <50 kV
Superficial beams – 50 to 150 kV
Orthovoltage – 150 to 500 kV
Beam quality described in terms of HVL of Al or Cu.
30 kV – 1 mm Al
100 kV – 3 mm Al
160 kV – 0.5 mm Cu
200 kV – 1 mm Cu

3. Protocols for calibration
NCRP Report 69
Uses exposure calibration factor Nx
AAPM TG-61
Uses air kerma calibration factor Nk
Nk = Nx· (W/e)air / (1-g)
(W/e)air = J/C (0.876 cGy/R)
g ≈ 0 for E < 300 kV

4. Energy dependence on calibration factor
6% between 40 kV and 300 kV
Tubes with same potential may have varying HVLs.
Chamber should be calibrated using beam of similar tube potential and HVL.

5. TG – 61 Methodology(s) Low energy (superficial): < 100 kV
Use in-air calibration method
Medium energy (orthovoltage): > 100 kV
Use in-phantom or in-air calibration method
Use in-air method if surface dose is desired

6. Measurement (M) M = Mraw Ppol Pion PTP Pelec
End effect (δt) may be significant and correction needed for irradiation time.

7. TG – 61 In-air method
Should use parallel-plate chamber for potentials < 70 kV.
Requires material (Table 1) to provide buildup and eliminate e- contamination.
Cylindrical chambers do not require buildup for in-air measurements.
Thimble thickness is 50 mg cm-2 or more.
Will require inverse-square correction for closed-cone systems.

8. TG – 61 In-air method Dw,z=0 = M· Nk · Bw · Pstem,air ·
Pstem,air accounts for change in photon scatter from chamber stem between chamber calibration and beam calibration.
Pstem,air = 1.0 if same field size is used for both chamber calibration and measurement.
Use guidelines in Sec. V C.7 to establish Pstem,air if same field size is not used

9. TG – 61 In-air method
is the ratio for water-to-air of the mean mass energy-absorption coefficients averaged over the incident photon spectrum.
Function of HVL
Table IV (Section B.1.1)
Backscatter factor Bw
Function of HVL, field diameter, and SSD
Table V (Section B.1.2)
Closed-end cones require additional correction. (Table VI)

10. TG – 61 In-phantom method
Calibration of medium-energy x rays can be performed in water.
Center of chamber at 2-cm depth.
If water-proofing is needed, use appropriate correction factors (Appendix B.2.3)
Psheath values listed in Table IX
Use talcum powder-free latex.

11. TG – 61 In-phantom method Dw,z=0 = M· Nk · PQ,cham · Psheath ·
PQ,cham accounts for change in chamber response due to change in beam characteristics between calibration and measurement, and the pertubation in photon fluence due to chamber in water.
Function of HVL and chamber type
Use Table VIII (Section B.2.2)

12. TG – 61 In-phantom method
is the ratio for water-to-air of the mean mass energy-absorption coefficients averaged over the photon spectrum at depth 2 cm and 10x10 field.
Function of HVL
Table VII (Section B.2.1)
If field size differs from 10x10:
Use Figure 3 to correct ratio of mass energy absorption coefficients
Use Figure 4 to correct for PQ, cham

13. TG - 61 Conversion to dose in other medium.
Table X lists the in-air ratios of mass energy-absorption coefficients of biological tissue to water.
Use in conjunction with in-air calibration method.
Conversion to dose in bone requres correction factor to Bw listed in Table XI.